The range and impact velocity of tube-launched projectiles can be enhanced with some form of power unit, for instance a solid fuel ramjet. This power unit uses oxygen from the surrounding air essentially for the whole of the fuel oxidation process. The propellant economy of the power unit is greatly improved in this way, in comparison with alternative rocket motors where the propellant is required to include an oxidation agent.
In principle, the ramjet is comprised of a hollow, tubular shell which is open at both ends and the front and rear openings of which function respectively as a combustion-air intake and a propulsion-gas outlet. Fuel is accommodated in the tubular shell between the openings, this fuel possibly having the form of a tubular body made for instance of a polymeric material and bonded to the outer hollow shell. In operation, the inner surface of the tubular fuel member, the combustion surface, burns and said surface will thus act as a combustion-chamber delimiting surface. One condition for initiating and continuing propulsion of the projectile is that a at least a given pressure prevails in the combustion chamber. This condition is achieved in projectiles, by launching the projectiles at a speed which correspond to a Mach number of at least about 2, therewith generating a sufficiently high so-called ram pressure in the combustion air flowing in through the intake. So that the combustion air will not prevent effective combustion of the aforesaid burning surface, a so-called flame holder is provided at the combustion chamber inlet in the most common non-rotary application of ramjets. The function of this flame holder is described in more detail herebelow.
When a ramjet of the aforedescribed kind is used with a spinning application, combustion difficulties occur because this spin results in rapid swirling of the gas in relation to the combustion chamber as it flows through said combustion chamber. The flame holders of known ramjets function satisfactorily when the gas flows along the combustion chamber but is impaired when the gas flow swirls in relation to the combustion chamber.
U.S. Pat. No. 4,796,534 describes a solution to this problem, wherein the tubular fuel member is made separate from the outer shell and is journalled in said shell for rotation about a longitudinal axis. Because of the force of inertia, the tubular fuel member will be carried by the rotation or spin of the projectile to only an insignificant extent, therewith greatly reducing rotation of the gas flow in relation to the combustion chamber. This solution, however, has the drawback of being complicated and expensive. Furthermore, the provision of a fuel-element support tube and the provision of journals reduce the useful load-carrying capacity of the projectile.